Polyimides, because of their outstanding thermal stability, have been favored for use in advanced engineering structures. In the past, polyimides were difficult to fabricate because of their insolubility in most of the more common solvents. The solubility problem was partially solved by using a polyamide-acid intermediate for product fabrication. During the final fabrication steps, imidization of the polyamide-acid is easily accomplished to give the desired end product. While this solved the solubility problem, it did not successfully solve a void problem caused by water liberated during imidization when the polyamide-acid was cured. The presence of voids in the final product is very undesirable because they reduce the mechanical properties of the product.
In U.S. Pat. No. 3,528,950, a solution to the void problem was offered. In this patent, a fully imidized prepolymer having reactive end groups was formed. In this way, the water of imidization was removed before final cure of the prepolymer during fabrication of the polyimide product. Although this substantially solved the void problem, solvent solubility was not as desirable as many fabricators would prefer.
Subsequently, U.S. Pat. No. 3,812,159 taught that a dianhydride monomer containing a phenoxyphenyl sulfone linkage could be used in the process taught by U.S. Pat. No. 3,528,950, and which would provide polyimides with improved solubility. The characteristics and synthesis methods for these polyimides are taught in U.S. Pat. No. 3,699,075.